Apparatus for annealing annular coils of sheet metal



c. CONE 2,600,094

APPARATUS FOR ANNEALING ANNULAR cons OF SHEET METAL June 10, 1952 FiledApril 15, 1948 IN VEN TOR Car/"0H Cone Patented June 10, 1952 IAPPARATUS FOR ANNEALING ANNULAR COILS OF SHEET METAL Carroll Cone, nearToledo, Ohio, assignor to 'Sur- I face Combustion Corporation, Toledo,Ohio, a

corporation of Ohio Application April 13, 1948, Serial No. 20,785

1 The present invention relates to a bell type furnace and a method ofoperation for annealing annular coils of sheet metal in a protectiveatmosphere maintained about the charge or stack of coils by a bell-typemetal cover disposed thereover. In a furnace of this type the heat forthe annealing operation is applied to the external side of the cover.Hence the heat is applied to the coils solely by radiation from thecover except for such heat as may be transferred from the cover to otherparts of the coils by circulation of the gaseous medium- (the protectiveatmosphere) within the cover. Conversely, cooling of the coils iseffected by dissipation of heat through the walls of said cover. Theobject of the present invention is to provide for the circulation ofsaid gaseous medium in such a way to provide for the heating and coolingof saidgaseous medium in such a way that the heat transferred by'convection should represent a substantial portion of the total heatrequired to be applied to coils during the annealing operation.

For a consideration of what I consider to. be novel and my invention,attention is. directed to the following specification and the claimsappended thereto.

In the accompanying drawing forming part of this specification:

Fig. 1 is a vertical section of an annealing furnace embodying thepresent, invention.

Fig. 2 is an enlarged fragmentary horizontal section on line 22 of Fig.1.

Fig. 3 is a plan view with a portion broken away of a channeled spacerelement adapted to be positioned between the annular coils to providegas passages therebetween.

Fig. 4 is a fragmentary end view of Fig. 3.

The furnace comprises a stand I on which a plurality of annular coils llof sheet metal may be supported one above the other with their cores invertical alinement whereby the charge is in the form of a stack having acentral gas passage. During the heating and cooling operations the coilsare protected against oxidation by a protective atmosphere in'a-bell-type metal cover l2 adapted to be disposed over the charge toform with the furnace base 13 on which the coil stand is supported asubstantially gas-tight heat-treating chamber. The seating portion ofthe inner cover l2 preferably comprises areinforcing band [9 of anglecross-section, having a horizontal portion welded gas tight to theside-wall and a vertical portion depending therefrom about which loosesand is disposed to reduce gas leakage from said chamber to a minimum.

3 Claims. (Cl. 263-40) The coil stand l0 comprisesa top plate [4 havinga central aperture, and a lower plate [5 separated from the top plate bya plurality of radially extending spacer ribs It to" provide a pluralityof radially extending gas passages between said plates. The lower platerests on a refractory pedestal which constitutes part of the furnacebase. A radial flow fan I! is mounted between the plates I l and J5coaxial with the aperture in the upper plate; hence the direction offlow ofprotective atmosphere is downwardly towards the fan and thenceradially therefrom. The 'fan is driven by an electric motor [8appropriately secured to the underside of the furnace base.

Heat for the annealing operation is provided by a portable heating hood20 disposed over the metal cover 12. On completion 'of the heatingoperation, the heating hood is removed but the protective cover 12 isleft in place; hence cooling of the coils is by dissipation of heatthrough the walls of said cover.- The heat producing means in the hood20 is of secondary importance but will ordinarily consist of heatradiating elements 21 such as internally fired tubes.

The furnace thus far described involves nothing new and is typical of aconventional bell-type annealing furnace. I

In the present invention, the adjacent ends of the coils are separatedfrom each other by an annular metal separator 22; hence any heating orcooling of the separator will result in similar heating or cooling ofthe ends of the coils in contact therewith. Each separator comprises aplurality of open-ended gas passages 23 between the inner and outer rimsthereof so that gaseous medium is free to flow through said passages. Inthe present invention the internal heat transfer area of the coilseparators is from 1.5 to 2.0 times the net area of the adjacent coilends, this increased area being provided by the combined area of theside walls 24 of said passages, it being understood that the height andnumber of said walls are .the determining factors.

The fan I! is sufficiently powerful to produce sufficient partial vacuumin the core of the stack of coils to cause the gaseous medium about thestack. to'flow throughseparator passages. 23 with avelocity between 2000and 3000 feet per minute. A closure plate 25 on top of the stackrestricts the flow of gaseous medium through and around the stack to thedesired minimum.

As best shown in Fig. 2, the sidewall of the protective cover 12 iscorrugated to provide a contact area for the gaseous medium inside ofthe cover that is between 1.75 and 2.25 times the projected area of saidwall thereby permitting a correspondingly greater amount of heat to beexchanged between said wall and said atmosphere. The furrows and ridgesof said corrugations are vertical. The use of a reinforcing band IQ. ofangle cross-section makes the vertically corrugated cover substantiallyrigid by preventing the corrugations from spread-- ingaccordion-fashion, and also provides a depending blade-type seal.

From the foregoing explanation it will beappreciated that there are twodistinctheat trans-.. fer processes involved in heating vor cooling thecoil charge. Radiation heat transfer, on the one hand, occurs primarilybetween the inner surface of the protective cover and the outer wraps ofthe coils, in quantity which varies directly with the projected innersurface of the cover, and at a rate which increases rapidly withtemperature. The rate of heating byradiation heat transfer is severely.limited by the rated heat transfer from. the. outer wraps of the coilsto the center thereof; Convection heat transfer, on theother hand,delivers heat energy primarily to or from the coil .ends in contact withthework separators used .inthe preferred arrangement. The quantityofheat transferred by convection varies withthe developed-rather thanthe projected area of the inside. coversurface, the effective surface orheat transfer area of .the coil separators, and the-power andefficiencyof the circulating fan.

Since convection heat transfer depends upon wiping heat offof theinnercover, and delivering it .tothe coilsas by a separator, a properdesign of r the inner cover together. with a correspondingly; carefulchoice of a coil separator anda circulating fan will be required to.obtain maximumbenefits from each.

Because of the basic differences in thetwo heating effects, the results:secured in temperature uniformity, heating-and cooling-time requirementsand :theshape f the time-temper atureqcurvesecured duringheating andcooling may be drastically modified by changingtherelative amounts ofheat;transferred-.by -radiation and convection. In g eneral,;thechange;to more heating; by convection increasesoveralL heat transfer; effectssince radiationheating; isnot necessarily. reduced: by the. expedientsused to promote. convection heat transfer;- However,- such: expedientsrepresent additional items 1 of investment and operating; cost, to theextent that.;.the determination ofithe. mostieconomical distribution ofthe heat :transfer burden between these. two processesbecomes a..veryimportant step-.in the design of coil heating equipment; hence asalready stated, Ihave found it desirable to; design the equipmenttosatisfy the following. empirical relations: a

1; The. internal heat transfer. area of coil sepa: rators. should totalbetween .1.5.-. and.2.01; times the net area of adjacent coilzends.

2.; The velocity of gas circulation througlnthe open passages. through.such' coil. separators shouldcbebetween 2000 and 3000feetper minute.

3. Theidevelopedinsidesurface area .of. the protective cover. shouldbe.between. 1.7 5: and 2.25 times the projected area-of thatsurfaca;

I;believe that with the-combination of heatin efiects:as;described,-,and-used-in accordancewith the, preferredratios defined above, Lobtairr.the mosteconomical operation with the bcstmetallurgioa-l results What Iclaim is:

1. Apparatus for annealing a stack of coils of sheet metal in aprotective atmosphere comprising, in combination, a base for supportingsaid stack. of coils, separators for spacing the coils in saidstack,said separators being. provided with passages for flow of saidprotective atmosphere between. the interior and the exterior of saidstack, a gas-tight cover for said stack having asingle verticallycorrugated cylindrical side Wall closely surrounding said stack, meanscomprising a power-driven fan for circulating said protective atmosphereat high velocity along the channels formed' by said corrugations andthroughthe interior of said stack, said atmosphere also flowing throughthe passages in said separators, and means for applying heat directly tothe exterior of the corrugations forming the channels. along which saidatmosphereis. circulated; said corrugated cylindrical side wallproviding-transfer of heat-tosaid coils by conduction; ata;substantially: greater rate than a; smooth cylindrical side wall, andprovidingtransfer of ;heat to .said-coils'by radiation at-substantially,as great. a ,rate as a smooth cylindrical sidewall.

2. Apparatusxaccording to. claim 1, comprising a. Supporting. ring;having. a horizontal flange secured to :the foot 101? the corrugatedcylindrical side wall ofthe-cover, and'havinga depending: cylindrical;portion servingas a; bladefor engagement with'thebase. to confine theprotective atmosphere within 1 the cover.

3. Inaprotectivecatmosphere annealing: furnace comprising. heating1means,:.a' base support-v ing a=stack of coils .of sheet metal;separators between said .coils that providepassages between the interiorand the exteriorof .said stack, and a high-capacity: power-drivenf anfor; circulating the. protective atmosphereithrough'. the interior ofsaid stack and through the passages in. said separators; the combinationtherewith of .a gastight cover, interposed between said heating.meanssandrsaid stack of coils,. havinga single vertically corrugatedcylindrical side wall closely REEERENCESZ CITED;

The following referencesare'of record in the-

